Fourteen plasma neutral amino acids are transported into brain of the adult rat by a single high-affinity system at brain capillaries. Transport affinity differs among the amino acids by 700 fold and is determined primarily by amino acid side chain hydrophobicity. At normal plasma concentrations, the transport system is saturated; each amino acid must compete for available transport sites. Competition makes the brain susceptible to imbalances in plasma amino acid concentrations. Neutral amino acid transport into rat brain decreases between 1 week and 3 months of age and then remains constant between 3 and 24 months. Changes in amino acid transport with age correlate with changes in brain protein synthesis. Amino acid transport is not induced or repressed by chronic hyperaminoacidemia. The nonmetabolizable amino acid, 1-aminocyclohexanecarboxylic acid, is transported by the cerebrovascular neutral amino acid carrier and may be a suitable in vivo probe of amino acid transport in humans using positron emission tomography. The kinetics of glucose transport across the blood-brain barrier can be described by a model with a single facilitated system. Accurate values of Vmax and Km were obtained for D-glucose, 2- deoxy-D-glucose and 3-0-methyl-D-glucose. Calcium concentrations in brain and CSF are maintained within 10% of control values after chronic changes of up to 50% in plasma Ca concentration. The transfer coefficient for 45Ca uptake into CSF is inversely related to plasma Ca concentration, demonstrating regulated transport of Ca at the blood-brain barrier.